The present invention relates to a pinhole removal device, more particularly to a device for removing pinholes produced in printing by lithographic press, intaglio press or the like.
When a nonabsorptive printing object such as a metallic plate, plastic plate, glass plate, etc. is printed by a lithographic press, intaglio press or the like, it is often the case that numerous pinholes are produced within a layer of ink, since numerous micro-bubbles are confined within the layer of ink of a form portion and the layer of ink is dried with the said micro-bubbles therein.
A serious problem exists in that pinholes within the layer of ink not only negatively affect the thickness of the printed surface of a name plate or the like, but, particularly in case of a printed board, anti-corrosion of the ink film which forms a printed circuit is decreased by such pinholes.
FIG. 1 is a typical illustration to exaggeratively show what is called the "cavitation theory" with respect to the formation of pinholes, wherein the ink film 12 of a form roller 11 is just transferred to the form portion of the form plate 14 mounted on a plate cylinder 13.
When the form roller 11 and the plate cylinder 13 contact each other and pass over the point A which is in contact with the form plate 14 by the rotation in the direction of the arrows at the same peripheral velocity respectively, as shown in FIG. 1, the pressure within the ink film 12 of the form roller 11 is rapidly increased and the first area without shear appears at the point B where the said pressure is maximized. On and after passing the contact portion between A and D (point C shows the center thereof), the said pressure is equal to the air pressure (gauge pressure stands zero), and the area is again without shear. After passing the said area, the pressure within the ink film 12 is reduced to a minimun below the air pressure, and then returns to the air pressure. Nearby the minimum point of the said pressure, a micro-hole is produced by the upward and downward tensions on the ink film 12. The said micro-hole grows up to a cavity and splits like cobwebbing, finally being transferred to the form portion of the form plate 14.
The layer of ink 12' thus split and transferred to the form portion of the form plate presents a rough surface composed of irregularity in the shape of micro-craters. The ink film 12' transferred to the form portion of the form plate 14 is to be levelled or smoothed by the surface tension with the lapse of time. However, the time it takes for the form portion of the form plate 14 to contact the printing object is so short that the said levelling does not sufficiently progress. Consequently, when the form portion contacts the smooth surface of the printing object, the air within the said micro-craters is still confined in the layer of ink to be transferred to the printing object, growing up to bubbles and finally forming pinholes. This is an outline of the cavitation theory with respect to the formation of pinholes.
It is understood by the said description that when printing object is a metallic plate or glass plate having no permeability, pinholes are easily produced in comparison with those having permeability like paper.
In order to reduce such pinholes, several attempts have been proposed such as a method for crushing and discharging bubbles confined within the layer of ink by increasing the printing pressure or by decreasing the printing speed, or a method for increasing the levelling effect of the ink film by employing some soft ink. However, any of those conventional methods has a fatal disadvantage that although they succeed in decreasing or reducing pinholes, the print contrast ratio or visibility of the printed image is inevitably reduced to a certain extent. Therefore, the said conventional methods are not the preferred means for removing pinholes.
It is, therefore, an object of the present invention to provide a pinhole removal device to solve the above-discussed problems and disadvantages inherent to printing by a lithographic press, intaglio press or the like by providing a levelling roller in contact with the form plate and/or the peripheral surface of a blanket cylinder before reaching the printing position, so that cratering or micro-craters on the ink film may be levelled or removed by the said levelling roller. More particularly the present invention contemplates a device for removing pinholes, wherein cratering or micro-craters on the ink film into which the air is taken to be confined in the form of bubbles within the layer of ink which is formed by transferring the said ink film of the form portion to the printing object from the form plate and/or the peripheral surface of the blanket cylinder, causing the formation of pinholes when the ink is dried and set, can be removed by the said levelling roller, thereby printing is successfully carried out without producing pinholes nor negatively affecting the print contrast ratio of the printed image.
Other objects, features and advantages of the present invention will become apparent in the course of the following description with the accompaning drawings.